Research on stress sensitivity of fracture-hole carbonate reservoirs under drilling fluid immersion

Abstract

Fracture-hole carbonate reservoirs feature the development of multi-scale fractures and dissolution holes. Invasion of drilling fluid into the reservoir and its prolonged interaction with the rock can readily induce a variety of damages, including stress-sensitive damage, consequently diminishing the productivity of oil and gas wells. Experiments were conducted on both dry/wet fractured rock samples and dry/wet fractured rock samples containing holes to investigate the mechanism of stress sensitivity in fracture-hole rocks subjected to fluid soaking. The results indicate that: (1) During the loading process, the highest damage rate to the permeability of fractured rock samples can reach up to 95%. The stress sensitivity coefficients for fracture-hole rock samples with half-hole depths of 2.5 mm, 5.0 mm, and 7.5 mm are 0.58, 0.41, and 0.25, respectively. The permeability damage rate of fracture-hole rock samples soaked with fluid is approximately 80%. (2) As the depth of the hole increases, the stress sensitivity coefficient decreases. The hole's depth is inversely proportional to the maximum stress, while being directly proportional to the maximum displacement of the rock sample. (3) Upon fluid soaking, pressure dissolution occurs within the fractured rock samples, shrinking the fracture width by 15.71–26.17%. (4) Following a 48-h immersion of the rock samples in an alkali solution, decalcification occurs and the pore structure is compromised, thus amplifying the degree of stress sensitivity.